This invention relates to an improved constant current welding power supply, and particularly a high capacity power supply that may be used occasionally in low current welding applications.
High capacity, constant current welding power supplies typically employ an SCR controlled bridge rectifier circuit to control the output current flow in response to a current sensor placed in the lead to the welding electrode. A typical power supply might provide in excess of 200 amperes of output current. SCR controlled bridge rectifiers are not fast acting, particularly when the power supply is being operated in a low power mode (in the order of 50 amperes). Thus, high capacity power supplies have not been suited for use when welding with small diameter electrodes where low power is required because they do not respond quickly enough to large current variations caused by such events as an occasional short circuit, the clearing of short circuits, changes in arc length caused by the operator, or changes in the burn off rate of the electrode. If the current level falls below a predetermined minimum level, for example, 30 amperes, then the arc may be momentarily extinguished, causing undesirable effects.
It is desirable, however, to be able to use a single power supply in multiple applications, and particularly, it is desirable to be able to use a high capacity constant current welding power supply for welding at constant current in low power applications.